Biomedical Engineering Reference
In-Depth Information
3.3
Structure, Morphology, and Evolution
The cross-sectional schematic showing the QDM-embedded structures grown on
the nanohole-and-mound template described above is shown in Fig.
3.2
. All growth
takes place in a Riber's 32P solid-source MBE chamber. Growth on epi-ready
(0 0 1)-GaAs substrate starts from thermal desorption of the surface oxide at 580
◦
C,
growth of a 300-nm GaAs smoothening layer at 610
◦
C, a brief pause to bring down
the substrate temperature to 500
◦
C before the growth of
x
ML of InAs QD seed
layer at a rate of 0.01 ML/s where
x
is between 1.8 and 2. This results in a QD
ensemble with typical base width of 30-50 nm and height of 5 nm. The onset of
QD formation, monitored in situ via the reflection high-energy electron diffraction
(RHEED) pattern, is confirmed by the appearance of the transmission spots which
subsequently develop into chevrons [
22
]. The substrate temperature is then brought
down further to 470
◦
C before the InAs QDs are partially capped with
y
ML of GaAs
where
y
is typically between 6 and 25. This ensures a successful formation of the
nanoholes whose depth can be varied approximately between 0.2 and 1 nm. Each
nanohole sits atop a nanomound elongated along the [1
1 0] direction as shown
in the lower atomic force microscopy (AFM) image in Fig.
3.2
. The elongation
observed is a result of asymmetrical In adatom diffusion lengths along the
−
directions which can be enhanced or suppressed by simply increasing or decreasing
the capping temperature [
15
]. To form the QDMs at the same temperature used
for capping,
z
ML of InAs is regrown on top of the nanohole-and-mound template
where
z
is typically between 1 and 2.5 for fully-developed molecules. For the sake
of convenience and precision, a QDM ensemble formed from
x
-ML InAs seed QDs,
y
-ML GaAs partial capping, and
z
-ML InAs overgrowth is hereafter referred to as
x
/
y
/
z
QDMs.
The QDMs are then capped with 100-nm GaAs where the first 10 nm is
grown at 470
◦
C and the remaining 90 nm is grown while ramping the temperature
up to 500
◦
C. For a single-layered QDM structure, the sample is quenched after
completing the 100-nm GaAs growth. For a bi-layer QDM structure, another cycle
of
x
/
y
/
z
QDM (with different values of
x
,
y
, and/or
z
from the first QDM layer) and
110
100-nm GaAs cap
QDM
2
100-nm GaAs spacer
QDM
1
300-nm GaAs buffer
(001)-GaAs substrate
Fig. 3.2
(
Left
) Cross-sectional structures of lateral QDMs grown on the nanohole-and-mound
template via the partial-cap and regrowth process. (
Right
)1
×
m AFM images of (
lower
image
) nanohole template and (
upper
) lateral QDMs. The arrows point along the [1
μ
m
0.5
μ
−
1 0] direction
